Ok, I already have a working Pandora battery and a MMS. I made the MMS with no problems.
Yet, when I popped in the battery while holding the L trigger the normal XMB booted up instead of the m33 menu!
I re-tried making my MMS over and over yet I still get the same result! Yes, I used Rain'sIPLtool as well too check my IPL space.
My PSP's a 1006, and my MS is Sony Memory Stick Pro Duo HIGH SPEED 4GB.

Problem definitely looks like it's the "Pandora" Battery. My wager is that you didn't really make a pandora.
If you made a proper pandora and put it in the PSP, then you should have booted into service mode. (as far as i know, you wouldn't have been sent to the XMB)
That's why making your MMS over and over, hasn't been fruitful. Have you looked at any youtube videos? They might shine some light on the process.

I don't think it's the battery's problem. Cause everytime I popped in the battery without any MS inside, it's a black screen with the green light on.
Thats how I know that it isn't the battery's problem! Anyway, what kind of MS do you guys suggest? SanDisk or Sony or others?

Sony MS pro was created in a partnership with SanDisk. Then Sony's need for a smaller flash memory card for pocket-sized digital cameras brought about the MS Duo Pro. Soon Sony and SanDisk will release the 2TB Memory Stick XC.

It automatically installed DC8 (5.00 m33-4). Put the 500.pbp in the same folder as the program. Direct it to your MS and all should be good. Be sure to check format MS and even backup if you have info you want to save first (or manually copy the files out). If that fails then yes it could be your MS or battery.